Plasma processing devices can be utilized to etch material away from a substrate formed from, for example, a semiconductor or glass. Plasma processing devices may contain a vacuum chamber that encloses plasma processing gases, which can be ionized and transformed into plasma. For example an energized source (radio frequency (RF), microwave or other source) can apply energy to the process gas to generate the plasma. In some plasma processing devices, the energy can be transmitted through a dielectric window that is formed through the vacuum chamber. Accordingly, the dielectric window can be subjected to heating induced by the electromagnetic energy. Moreover, the heating can be localized to specific regions of the dielectric window due to variations in electromagnetic energy caused by process conditions. There can be two sources of heating of the dielectric window. First, the dielectric properties of the window (tangent-δ) may result in the direct absorption of RF or microwave power. Second, the plasma created by the energized source can indirectly heat the window. Moreover, the heating can be evenly distributed across the dielectric window or localized to specific regions of the window due to the design of the source (antenna construction, etc) and plasma conditions.
Heat energy can be removed from dielectric windows passively (i.e. no cooling device) or with a cooling device such as a liquid cooling system or a fan cooling system. Liquid cooling systems can be efficient but are more expensive than passive cooling or fan cooling systems. Moreover, liquid cooling systems are more difficult to implement in an environment subjected to electromagnetic energy. For example liquid cooling can cause localized cooling resulting in thermal gradients and thermal cracking. The dielectric properties for the liquid are different to the surrounding ceramic resulting in non-uniform transmission of the RF power. For example, the liquid may be conducting which would result in the dissipation of RF power within the liquid. The liquid may be subject to nucleation and can be difficult to contain within the cooling system.
Fan cooling systems can be utilized for cooling of dielectric windows such as, for example, via convection. However, fan cooling systems can be inefficient and difficult to apply to localized regions of relatively high heat load induced by the energized source in a dielectric window. Specifically, fan cooling systems suitable for use with plasma processing devices are ineffective for heat removal when subjected to high back pressure. For example, fan cooling systems may stall and fail to provide sufficient air flow for cooling when subjected to back pressures of about 0.5 in-H2O or more.
Accordingly, a need exists for alternative devices for cooling dielectric windows of plasma processing devices.